1. Field of the Invention
This invention pertains to a system and method for efficient ion transport of ions having a wide range of masses. More specifically, a DC voltage gradient is generated which does not suffer from mass discrimination.
2. State of the art
One mass spectrometer subsystem which precedes a mass spectrometer is an ion transport system. This application incorporates by reference the materials in U.S. Pat. application Ser. No. 08/751,509 which teaches an improved mass spectrometer.
One example of the state of the art in ion transport via an electrode path is accomplished as shown in FIG. 1A. Here, a system 8 is comprised of four electrodes 10, where one electrode 10 is obscured by another in this view. The obscured electrode is visible in FIG. 1B when the system 8 is viewed on end. In FIG. 1A, the path 12 an ion 14 travels is shown as indicated to be generally along with and parallel to a lengthwise quadrapole axis 18 of the electrodes 10. The electrodes 10 are charged with an RF component. The RF component is provided so that ions are confined in the radial direction relative to the lengthwise axis 18 of the quadrapole system 8.
The system 8 shown in FIG. 1A is known as an RF quadrapole because of the four electrodes 10 which generate the RF field for confining ions in the radial direction. However, other electrode configurations are also present in the state of the art, such as six (hexapole) or eight (octapole) electrode systems. All function similarly in that the systems provide confinement in the radial direction. However, for ions 14 traveling near the axis of the system 8, the effect of higher order RF fields created by a greater number of electrodes is minimal. That is, the electrodes 10 exert their focusing action further from the axis 18 of the system 8. Therefore, while the drawbacks associated with a quadrapole system 8 will be examined closely, it should not be construed as an indication that higher order RF fields provide any significant differences relative to the quadrapole which is discussed in detail hereinafter.
FIG. 1B is provided to show that the electrodes 10 (FIG. 1A) are arranged such that they are generally positioned at four corners of a square. This means that the distance from any electrode 10 to the nearest two electrodes is generally equidistant for each of the electrodes.
Generating a DC axial field gradient is useful when it is desirable to accelerate ions axially along the quadrapole axis 18. The DC field gradient is also useful in overcoming drag forces arising from the presence of background gas which may be present along the ion path.
A first method for generating the DC axial field gradient is through biasing endcaps 6 of the quadrapole system 8. Endcaps 6 provide the DC bias or field gradient necessary to propel the ions 14 along the path 12, while the ions 14 are confined generally to the center of the system 8 by the RF fields. Endcaps 6 are typically conductive plates which have a DC voltage applied thereto.
FIG. 2 is provided to show a perspective view of a distal end of the system of FIG. 1A in the prior art for generating a DC axial field using a single large endcap plate 4 in the shape of a disk. In order to create the DC voltage gradient, a different DC voltage must be applied to each endcap plate 4. FIG. 2 shows only one endcap plate 4, and another endcap plate 4 (not shown) is thus disposed at a proximal end of the system 8. Each endcap plate 4 includes an aperture 2 generally at a center point to allow entry or exit of an ion 14 therethrough. A problem with endcaps, however, is that they generate DC fields only at the proximal and distal ends of the system 8. Consequently, the DC field along a significant length generally near a midpoint of the system 8 is disadvantageously weak.
Another method of improving ion transport performance is to generate stronger DC field gradients. This is accomplished by tilting or tapering the electrodes 10 in conjunction with a DC biasing scheme. Tilting and tapering electrodes 10 enables the DC axial field to have a greater influence on ions 14 by bringing the DC axial field physically closer to the ion path 12 (FIG. 1).
FIGS. 3A and 3B illustrate this method of using tilted electrodes as taught in the prior art. FIG. 3A shows the electrodes 20, 22, 24 and 26 at an arbitrarily selected distal end of the system 8. FIG. 3B shows the same electrodes 20, 22, 24 and 26 at a proximal end of the system 8. By changing from a "flattened diamond" shape in FIG. 3A to a "thin diamond" shape in FIG. 3B, a DC field gradient is created with reference to the quadrapole axis 18. The DC field gradient is generated by a DC bias applied between pairs of electrodes 20, 22, 24 and 26. The applied RF voltages are indicated within the electrodes 20, 22, 24 and 26. The polarity of the DC axial gradient voltages are indicated outside each of the same electrodes.
A significant drawback to the method described above is that in addition to an axial DC electrical field, a quadrapolar DC field is disadvantageously generated. The effect of the quadrapolar DC field is summarized as introduction of mass discrimination. More specifically, mass/charge discrimination occurs in that a narrower range of ions can be transported via the electrodes 20, 22, 24 and 26, where the range of ions is determined by the mass thereof. To increase an axial acceleration field, a stronger DC field gradient is required. However, the disadvantage is that increasing the strength of the DC gradient results in a corresponding increase in the undesirable quadrapolar DC field.
While a quadrapolar configuration which only has radio frequency energy applied thereto has a theoretical low ion mass cut-off, there is no high ion mass cut-off. However, the addition of the quadrapolar DC field introduces a high ion mass cut-off. In applications requiring a large passband, this high ion mass cut-off is unavoidable in the prior art. This is because the magnitude and sign of the quadrapolar DC field varies with axial position. Therefore, it is not possible to compensate by superpositioning an additional quadrapolar DC field on the system 8.
Accordingly, it would be an advantage over the prior art to reduce mass discrimination by eliminating the quadrapolar DC field. It would be a further advantage to be able to manipulate the RF quadrapolar, the DC quadrapolar and the DC axial fields independently of each other.